The attenuation of a gamma beam is defined as the fraction of rays which come through an absorber without interacting. Of the 6 gammas represented (NB: there are billions of gamma rays in a sample), only A and C emerge unaffected. B, E and F are totally absorbed by the screen, whereas gamma ray D is partially absorbed. Gamma D is not counted with A and C as it emerges from the absorber as a Compton photon, travelling in a different direction with a lower energy. These Compton photons are counted, however, when dealing with ‘absorption’ as opposed to ‘attenuation’ of gamma rays.